This invention relates generally to high-power solid-state lasers and, more particularly, to techniques for reducing birefringence in solid-state lasers. Solid-state lasers with an average power up to 100 W (watts), and even higher powers, are needed in a variety of military, industrial and commercial applications, including X-ray photolithography, laser machining and drilling, space and underwater communication, and medical applications.
The brightness of a laser beam is proportional to the average power and is inversely proportional to the square of the beam quality, where the beam quality is in turn defined in relation to a diffraction-limited beam, i.e., a diffraction-limited beam has an ideal beam quality of 1.0. A worse beam quality of, say, 1.5 results in a brightness of 1/(1.5).sup.2 or 44.4% of the brightness of the diffraction limited beam. Since the brightness falls off in proportion to the square of the beam quality, it is extremely important to control the beam quality if high brightness is a design goal. The parent application cross-referenced above was principally concerned with various structural features that improved beam quality and brightness. Another aspect of the same problem is birefringence in the optical components used to generate the beam. Birefringence can be optically filtered from the output beam, and discarded to avoid equipment damage, but the removal of the birefringence component results in a lower output power and brightness. Therefore, it is more desirable to compensate for birefringence rather than to remove it from the output beam.
A number of laser architectures disclosed in various prior patents use a phase conjugated master oscillator power amplifier (PC MOPA) configuration, but still fail to produce a desirably bright beam, or have other drawbacks. The parent application cross-referenced above discloses and claims a high brightness solid-state laser source that includes a master oscillator, a solid-state amplifier and a phase conjugation cell positioned to receive the amplified beam from the solid-state amplifier and to reflect the beam in phase conjugated form back into the solid-state amplifier for a second pass. Aberrations introduced in the amplifier during the first pass are practically canceled during the second pass and the amplified beam has both high power and good beam quality. Although this laser source operates satisfactorily in many applications, it still suffers from birefringence as the amplifier heats up in operation. At higher temperatures, the amplifier crystal is thermally stressed and becomes anisotropic, exhibiting different indices of refraction along its different axes. Consequently, light propagates through the crystal at different speeds along the different axes, resulting in birefringence. Light emerging from the amplifier is no longer linearly polarized, but in general is elliptically polarized to some degree.
As described in the cross-referenced patent application, birefringence components in the amplified beam subject the master oscillator to possibly serious damage if they are reflected back into the master oscillator. One way to avoid this problem is to install a Faraday rotator, referred to in the prior description as a Faraday isolator, next to the master oscillator. The Faraday rotator protects the master oscillator from energy leaking through a polarizer used to couple an output light beam from the laser source. In theory, the polarizer reflects the light beam returning from the amplifier and thereby couples it out of the laser source. However, any birefringence components in the return beam will pass through the polarizer and back into the master oscillator, which can be seriously damaged as a result. The Faraday rotator rotates the polarization direction by 45.degree. on each pass, with the result that the birefringence components are effectively dumped out of the system by the polarizer. This is a known technique for removing birefringence components. Since the components are effectively discarded, they represent a loss in the power of the output laser beam.
The foregoing is only one example of an optical system in which unwanted birefringence arises. More generally, there is a need for birefringence compensation in a variety of optical systems.
It has been proposed by I. D. Carr and D. C. Hanna, in a paper entitled Performance of a Nd:YAG Oscillator/Amplifier with Phase-Conjugation via Stimulated Brillouin Scattering, Appl. Phys. B 36, 83-92 (1985), that a Faraday rotator may be positioned between an amplifier and a mirror in a master oscillator power amplifier (MOPA) system, to reduce birefringence effects. Birefringence effects manifest themselves in the form of two light beams that, because of anisotropic crystalline properties of the amplifier as the temperature increases, travel at different velocities. The amplifier crystal exhibits a lower index of refraction, and a correspondingly higher speed of transmission in one direction, as compared with an orthogonal direction in which the index is higher and the speed of transmission lower. If light from the amplifier is rotated 45.degree. by the Faraday rotator and then an additional 45.degree. during a return pass through the Faraday rotator, the resulting light beam has its "fast" and "slow" components interchanged. A second pass through the amplifier effectively nullifies the birefringence. Intuitively, one can appreciate this effect by considering that the "fast" component of the beam takes the "slow" path through the amplifier crystal on the return pass. Likewise, the "slow component of the beam takes the" "fast" path. The net effect, in theory, is to nullify the birefringence components in the amplified beam.
A practical difficulty with this approach to compensating for birefringence is that it cannot be used to advantage with one of the most commonly used phase conjugating mirrors, the stimulated Brillouin scattering (SBS) cell. In an SBS cell, containing a suitable SBS medium, such as liquid freon or gaseous nitrogen, the SBS process reverses the wavefront of an input beam. (Portions of the wavefront that were lagging become leading, and vice versa.) Aberrations impressed on the wavefront during the first pass through the amplifier are, therefore, negated and virtually removed during the second pass after reflection from the SBS cell. The SBS cell operates most effectively when the incident light is circularly polarized and the SBS medium is subject to optical breakdown if linearly polarized light is used. Therefore, use of a Faraday rotator to compensate for birefringence limits the effectiveness of the SBS cell because the incident beam is predominantly linearly polarized.
Another practical difficulty with Faraday rotators is that they do not always provide a desired angle of rotation of the direction of polarization. If the nominal rotation angle is 45.degree., it is not uncommon for the actual rotation angle to be in error by a few degrees, and for spatial variations to occur over the aperture of the rotator. After two passes through the rotator, the expected rotation angle of 90.degree. may be in error by as much as .+-.5.degree. or more. Clearly, this inaccuracy results in less than complete birefringence compensation.
In the laser source described in the cross-referenced application, a quarterwave plate located next to the SBS cell serves to produce circularly polarized light. More specifically, on the first pass through the quarter-wave plate the linear polarization of the beam is converted to circular polarization. On the return pass, the circularly polarized beam is converted back to linearly polarized light, but with a polarization direction orthogonal to that of the original beam. The orthogonal relationship between the forward and return beams is used to outcouple light by means of a polarizer.
Although a quarter-wave plate produces circularly polarized light, which is desirable for operation of an SBS phase conjugating mirror, the plate does not provide birefringence compensation. A significant birefringence component finds its way back to the master oscillator, where it must be removed to avoid equipment damage. Therefore, it will be appreciated that there is still a need for improvement in techniques for birefringence compensation in laser sources having medium to high power and good beam quality. The present invention satisfies this need.